This invention relates to improvements in apparatus used for electrodialysis of aqueous solutions containing ionizable inorganic or organic constituents which are to be removed from the solution or suspension. In particular, the improvements are in the electrodes which are situated in the end compartments through which it is the normal practice to circulate anolyte and catholyte solutions in electrodialyzers.
Generally, electrodialysis apparatus comprises a multichamber cell in which depletion chambers and concentrating chambers are arranged in alternating order, which chambers are defined by a plurality of spaced alternatingly arranged membranes at least some of which are selectively permeable to ions. The membranes are mounted between planar gaskets which define the chambers and seal the peripheries of the membranes. A chamber at the end of the cell adjacent one membrane has one or more anodes in it and a chamber at the other end adjacent another membrane, has one or more cathodes in it. The membranes, gaskets and end chambers are arranged in a stack which is secured by means which provide endwise compression. Anolyte and catholyte solutions are usually conducted continuously through the anode and cathode chambers, respectively, during operation of the apparatus. The solution from which ions are to be extracted is flowed through the depletion chamber and the solution in which the ions are to be collected or concentrated is flowed through the concentration chambers. The ions are transferred, depending on their polarity, through the ion selective membranes under the influence of an electric potential applied to the electrodes between which an electric current passes in series across the membranes and the chambers defined between them.
The anodes and cathodes are made of various material such as platinum, stainless steel and carbon. Most frequently, according to prior practice, the electrodes in the end compartments constitute plates which degrade or otherwise erode in time and have to be replaced. In prior art apparatus, this meant that the stack had to be drained and dismantled in order to gain access to the electrodes for replacing them. This is not only an inconvenient procedure but it is time consuming and reduces the productivity of the apparatus.